Electrokinetic phenomena include electromigration, electroosmosis, and electrophoresis. Electroosmosis is defined as the mass flux of a fluid containing ions through a stationary porous medium caused by the application of an electrical potential. The fluid moves through the voids in the porous medium (e.g. soil), called pores, where the pore walls have at least a slight electrical charge, either positive or negative. Each pore has a thin layer of charged fluid next to the pore wall having a typical thickness of between about 1 and about 10 nanometers. The thin layer of charged fluid next to the pore wall is present to neutralize the charge on the surface of the soil particle that forms the pore wall. Fluid movement occurs in soil pores because of the charge interaction between the bulk of the liquid in the pore and the thin layer of charged fluid next to the pore wall. Under the influence of a DC electric field, the thin layer of charged fluid moves in a direction parallel to the electric field. Large amounts of the bulk liquid may be transported along with the thin layer of charged fluid as well as contaminants or other species contained within the liquid.
Electromigration is defined as the mass flux of a charged ionic or polar species within a liquid or solution from one electrode to another electrode. Electromigration and electroosmosis may occur simultaneously or independently and are the dominant mechanisms through which conventional electrokinetic transport processes occur.
Electroosmosis has been used as a method for dewatering soils and sludges. Furthermore, electrokinetic metal recovery has been used as a method for mining metals, such as copper. These processes involved inserting electrodes enclosed within porous enclosures or wells into the ground. These enclosures are filled with an electrolyte solution, typically an acid solution.
Recently electrokinetic transport of materials has been applied to the electrokinetic remediation of contaminants in soil. Electrokinetic remediation, frequently referred to as either electrokinetic soil processing, electrochemical decontamination or electroreclamation, uses electrical currents applied across at least a pair of electrodes placed in the ground to extract radionuclides, heavy metals, certain organic compounds, or mixed inorganic species and organic wastes from soils and slurries and the like. The contaminants in the soil are moved under the action of the electrical field to wells where they are then pumped out.
During electrokinetic processing, water in the immediate vicinity of the electrodes is electrolyzed to produce H+ ions at the anode and OH-- ions at the cathode, causing the pH of the soil to change, according to the following equations.
Anode Reaction EQU 2H.sub.2 O.fwdarw.O.sub.2 +4e.sup.- +4H.sup.+ Equation (1)
Cathode Reaction EQU 2H.sub.2 O+2e.sup.-.fwdarw.H.sub.2 +2OH.sup.- Equation (2)
If the ions produced are not removed or neutralized, these reactions lower the pH at the anode and raise the pH at the cathode. Protons formed at the anode migrate towards the cathode and can aid contaminant removal by increasing metal extraction. In contrast, the hydroxyl ions formed at the cathode do not migrate through soil pores as efficiently as protons and can increase the soil pH in the cathode region, as high as a pH of 12, and cause deposition of insoluble species, thereby forming regions of high electrical resistivity. These pH changes can have a significant effect on the soil zeta potential as well as solubility, ionic state and charge, and the level or adsorption of the contaminants. It is, therefore, desirable to control the pH of the fluids in the vicinity of the electrodes, as well as the pH of the fluid transported between the electrodes.
The electrical charge on a soil particle is important in the transport of pore liquid by electroosmosis. Soil particles typically have an overall negative charge. The origin of the charge on the soil when in contact with an aqueous solution results from a number of effects, including chemical and physical adsorption and lattice imperfections. The saturating liquid composition and its pH are critical to the soil surface charge. Several reports have shown that an acid front moving through the soil in the direction of anode to cathode may reduce the electroosmotic flow and eventually stop the process. The electroosmotic flow slows down because the excess H+ ions neutralize the charge on the soil particles, thus decreasing ionically driven fluid flow.
It would be beneficial to have a process capable of monitoring rates of electroosmotic flow and adjusting the charge on the soil to enhance electroosmotic flow, either on a continuous or semi-continuous basis or when the charge on the soil causes the electroosmotic flow to slow down or stop. In areas with highly porous mediums, such as sand, it would be desirable to effectively control the flow of fluids there through and overcome the gravity-induced downward drainage of the fluids. Because, the voltage drop across the well wall and the soil effects the rate of electroosmotic flow, depending on the type of soil being remediated, it would be useful if the voltage drop could be adjusted to control or improve electroosmotic flow through a porous medium.